What are Copy Elision and Return Value Optimization in C++?

As you know, optimizing performance is important in C++ when you are dealing with the object creation and copying. Copy elision and Return Value Optimization (RVO) are the two optimization techniques that avoid unnecessary object copies and improve efficiency. These optimization techniques play an important role in C++ programming. In this article, we will discuss copy elision and RVO, their differences, and compilers supporting copy elision and RVO.

Table of Contents:

• Copy Elision in C++
  • When Does Copy Elision Occur?
• Return Value Optimization (RVO) in C++
  • How Does RVO Work?
• Compilers Supporting RVO and Copy Elision
• Difference Between NRVO and RVO in C++
• Benefits of Copy Elision and Return Value Optimization (RVO)
• Conclusion

Copy Elision in C++

Copy elision is the ability of the compiler to optimize away the unnecessary copy operations when an object is created, and due to this optimization ability compiler can create objects directly in the memory location where they will be used instead of creating them in a temporary memory location and then copying them.

When Does Copy Elision Occur?

Below are the following scenarios in which copy elision occurs:

1. Return Value Optimization (RVO): When a function returns an object by value, the compiler creates that object directly in the memory location where it will be received without the need for a temporary object.
2. Named Return Value Optimization (NRVO): When a function returns a named local object, then NRVO is applied. The compiler can optimize the copy by creating the object in the memory allocated for the return value.

Example:

Output:

The code shows how by returning a local object obj from the createObject() a copy elision occurs. Normally, this would call the copy constructor, but due to copy elision, the compiler creates or constructs obj directly in myObj in the main() function, which avoids the copy constructor call and only gives the constructor and destructor as output.

Return Value Optimization (RVO) in C++

Return Value Optimization (RVO) is an optimization technique that helps the compiler to create the return value of a function directly in the memory location where it will be used instead of creating a temporary object and then copying it. Also, this optimization reduces the overhead related to copy construction and helps improve performance.

How Does RVO Work?

When a function returns an object by value, the compiler creates a temporary object to hold the return value. With RVO, the compiler finds that temporary objects are unnecessary, and it directly creates the return values in the space allocated for the variable that receives those return values, which avoids the need for the copy constructor.

Example:

Output:

The code shows Return Value Optimization (RVO) in which the compiler can create an obj directly in the memory of myObj that eliminates the need for a copy constructor call and only calls the constructor and destructor.

Compilers Supporting RVO and Copy Elision

Difference Between NRVO and RVO in C++

Benefits of Copy Elision and Return Value Optimization (RVO)

Below  are a few benefits of copy elision and return value optimization in C++:

• Improved Performance: It improves performance by avoiding object copies and reducing the execution time.
• Reduced Use of Memory: It avoids creating temporary objects and reduces unnecessary memory usage. 
• Better Resource Management: It minimizes the constructor, copy constructor, and destructor calls, thus providing efficient resource management.
• Better Cache Utilization: Copy elision and RVO help to reduce the unnecessary memory operations, which increases the CPU cache efficiency.
• Simplifies Code Optimization: It helps you to return objects by value without any performance reduction.

Conclusion

Copy Elision and Return Value Optimization are important in C++ for compiler optimization. They help to improve performance and avoid unnecessary copies of an object. RVO avoids creating the temporary objects, and copy elision makes sure that the functions return values more efficiently. So, by understanding and using these techniques, you can easily write a C++ program without any errors and with high performance.

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